Shanan E. Peters

Phanerozoic trends in the global diversity of marine invertebrates.

It has previously been thought that there was a steep Cretaceous and Cenozoic radiation of marine invertebrates. This pattern can be replicated with a new data set of fossil occurrences representing 3.5 million specimens, but only when older analytical protocols are used. Moreover, analyses that employ sampling standardization and more robust counting methods show a modest rise in diversity with no clear trend after the mid-Cretaceous. Globally, locally, and at both high and low latitudes, diversity was less than twice as high in the Neogene as in the mid-Paleozoic. The ratio of global to local richness has changed little, and a latitudinal diversity gradient was present in the early Paleozoic.

Biodiversity in the Phanerozoic: a reinterpretation

Abstract Many features of global diversity compilations have proven robust to continued sampling and taxonomic revision. Inherent biases in the stratigraphic record may nevertheless substantially affect estimates of global taxonomic diversity. Here we focus on short-term (epoch-level) changes in apparent diversity. We use a simple estimate of the amount of marine sedimentary rock available for sampling: the number of formations in the stratigraphic Lexicon of the United States Geological Survey. We find this to be positively correlated with two independent estimates of rock availability: global outcrop area derived from the Paleogeographic Atlas Project (University of Chicago) database, and percent continental flooding. Epoch-to-epoch changes in the number of formations are positively correlated with changes in sampled Phanerozoic marine diversity at the genus level. We agree with previous workers in finding evidence of a diversity-area effect that is substantially weaker than the effect of the amount of preserved sedimentary rock. Once the mutual correlation among change in formation numbers, in diversity, and in area flooded is taken into consideration, there is relatively little residual correlation between change in diversity and in the extent of continental flooding. These results suggest that much of the observed short-term variation in marine diversity may be an artifact of variation in the amount of rock available for study. Preliminary results suggest the same possibility for terrestrial data. Like the comparison between change in number of formations and change in sampled diversity, which addresses short-term variation in apparent diversity, the comparison between absolute values of these quantities, which relates to longer-term patterns, also shows a positive correlation. Moreover, there is no clear temporal trend in the residuals of the regression of sampled diversity on number of formations. This raises the possibility that taxonomic diversity may not have increased substantially since the early Paleozoic. Because of limitations in our data, however, this question must remain open.

Determinants of extinction in the fossil record.

The causes of mass extinctions and the nature of biological selectivity at extinction events are central questions in palaeobiology. It has long been recognized, however, that the amount of sedimentary rock available for sampling may bias perceptions of biodiversity and estimates of taxonomic rates of evolution. This problem has been particularly noted with respect to the principal mass extinctions. Here we use a new compilation of the amount of exposed marine sedimentary rock to predict how the observed fossil record of extinction would appear if the time series of true extinction rates were in fact smooth. Many features of the highly variable record of apparent extinction rates within marine animals can be predicted on the basis of temporal variation in the amount of exposed rock. Although this result is consistent with the possibility that a common geological cause determines both true extinction rates and the amount of exposed rock, it also supports the hypothesis that much of the observed short-term volatility in extinction rates is an artefact of variability in the stratigraphic record.

Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion

The transition between the Proterozoic and Phanerozoic eons, beginning 542 million years (Myr) ago, is distinguished by the diversification of multicellular animals and by their acquisition of mineralized skeletons during the Cambrian period. Considerable progress has been made in documenting and more precisely correlating biotic patterns in the Neoproterozoic–Cambrian fossil record with geochemical and physical environmental perturbations, but the mechanisms responsible for those perturbations remain uncertain. Here we use new stratigraphic and geochemical data to show that early Palaeozoic marine sediments deposited approximately 540–480 Myr ago record both an expansion in the area of shallow epicontinental seas and anomalous patterns of chemical sedimentation that are indicative of increased oceanic alkalinity and enhanced chemical weathering of continental crust. These geochemical conditions were caused by a protracted period of widespread continental denudation during the Neoproterozoic followed by extensive physical reworking of soil, regolith and basement rock during the first continental-scale marine transgression of the Phanerozoic. The resultant globally occurring stratigraphic surface, which in most regions separates continental crystalline basement rock from much younger Cambrian shallow marine sedimentary deposits, is known as the Great Unconformity. Although Darwin and others have interpreted this widespread hiatus in sedimentation on the continents as a failure of the geologic record, this palaeogeomorphic surface represents a unique physical environmental boundary condition that affected seawater chemistry during a time of profound expansion of shallow marine habitats. Thus, the formation of the Great Unconformity may have been an environmental trigger for the evolution of biomineralization and the ‘Cambrian explosion’ of ecologic and taxonomic diversity following the Neoproterozoic emergence of animals.

Phanerozoic Earth System Evolution and Marine Biodiversity

Environmental factors, more so than sampling biases, drive trends in biological evolution observed in the fossil record. The Phanerozoic fossil record of marine animal diversity covaries with the amount of marine sedimentary rock. The extent to which this covariation reflects a geologically controlled sampling bias remains unknown. We show that Phanerozoic records of seawater chemistry and continental flooding contain information on the diversity of marine animals that is independent of sedimentary rock quantity and sampling. Interrelationships among variables suggest long-term interactions among continental flooding, sulfur and carbon cycling, and macroevolution. Thus, mutual responses to interacting Earth systems, not sampling biases, explain much of the observed covariation between Phanerozoic patterns of sedimentation and fossil biodiversity. Linkages between biodiversity and environmental records likely reflect complex biotic responses to changing ocean redox conditions and long-term sea-level fluctuations driven by plate tectonics.

Environmental determinants of extinction selectivity in the fossil record.

The causes of mass extinctions and the nature of biological selectivity during extinction events remain central questions in palaeobiology. Although many different environmental perturbations have been invoked as extinction mechanisms, it has long been recognized that fluctuations in sea level coincide with many episodes of biotic turnover. Recent work supports the hypothesis that changes in the areas of epicontinental seas have influenced the macroevolution of marine animals, but the extent to which differential environmental turnover has contributed to extinction selectivity remains unknown. Here I use a new compilation of the temporal durations of sedimentary rock packages to show that carbonate and terrigenous clastic marine shelf environments have different spatio-temporal dynamics and that these dynamics predict patterns of genus-level extinction, extinction selectivity and diversity among Sepkoski’s Palaeozoic and modern evolutionary faunae. These results do not preclude a role for biological interactions or unusual physical events as drivers of macroevolution, but they do suggest that the turnover of marine shelf habitats and correlated environmental changes have been consistent determinants of extinction, extinction selectivity and the shifting composition of the marine biota during the Phanerozoic eon.

Predation upon hatchling dinosaurs by a new snake from the late Cretaceous of India.

A new snake from Upper Cretaceous rocks in India is found with hatchling sauropod dinosaurs, demonstrating that large, gape-limited snakes were probably capable of taking in moderate-sized vertebrate prey.

Genus extinction, origination, and the durations of sedimentary hiatuses

Abstract Short-term variations in rates of taxonomic extinction and origination in the fossil record may be the result of true changes in rates of turnover, variable rates of fossil preservation, or some combination of the two. Here, positive extinction and origination rate excursions among Phanerozoic marine animal genera are reexpressed in terms of the amount of normal, background time they represent. In addition to providing a background-adjusted calibration of rate intensities, this reexpression determines the durations of sampling gaps that would be required to explain entirely all observed rate excursions as sampling artifacts. This possibility is explored by analyzing a new compilation of the timing and duration of sedimentary hiatuses in North America. Hiatuses spanning more than approximately one million years (Myr) in the marine sedimentary rock record have a mean duration of 73 Myr. There are two major hiatus types—those that form in response to long-duration tectonic cycles and that bound the major Sloss-scale sequences (mean duration 200 Myr), and those that form in response to shorter-duration changes in sediment accommodation space and that occur within major Sloss-scale sequences (mean duration less than 23 Myr). The latter are approximately exponentially distributed and have a mean duration that is comparable to the mean duration of intervening sedimentary rock packages. Although sedimentary hiatuses are generally long enough in duration to accommodate the hypothesis that short-term variations in rates of genus origination and extinction are artifacts of sampling failures at major unconformities (“Unconformity Bias” hypothesis), the observed evolutionary rates are not correlated with hiatus durations. Moreover, hiatuses that follow the major mass extinctions are not long in comparison to most other non–mass extinction intervals. These results do not support the hypothesis that hiatuses at major unconformities alone have artificially clustered genus first and last occurrences, thereby causing many of the documented statistical similarities between the temporal structure of the sedimentary rock record and macroevolutionary patterns. Instead, environmental changes related to the expansion and contraction of marine environments may have been the primary forcers of both biological turnover and the spatio-temporal pattern of sediment accumulation. Fully testing this “Common Cause” hypothesis requires quantifying and overcoming lingering taxonomic, biostratigraphic, facies, and numerous other biases that are both inherent in geologic data and imposed by imperfect knowledge of the fossil record.

Macrostratigraphy of North America

The geological record is a three‐dimensional mosaic of gap‐bound rock bodies that, at any given scale of temporal resolution, were emplaced more or less continuously. At any geographic location, the irregular alternation of processes responsible for the formation and destruction of rock bodies results in the preservation of hiatus‐bound rock packages that have a distinct time of first occurrence (base, or oldest portion), a time of last occurrence (top, or youngest portion), and a suite of defining characters (e.g., lithologies, thickness, fossils, etc.). Many important aspects of the geologic record can be quantified by compiling the stratigraphic ranges of such gap‐bound rock packages. These include the quantity and spatial and temporal distribution of preserved rock, rates of rock formation, sequence stratigraphic architecture, and area‐weighted rates of expansion and contraction/erosional truncation of rock emplacement settings. This approach to characterizing the rock record is scalable, permitting application to records encompassing entire continents and hundreds of millions of years as well as individual basins and geologically short time intervals. To illustrate the utility of this approach and to provide a new high‐resolution analysis of the temporal structure of the geologic record, gap‐bound rock packages in the continental United States and southern Alaska were compiled directly from the American Association of Petroleum Geologists Correlation of Stratigraphic Units of North America (COSUNA) charts. The COSUNA charts were assembled at a temporal resolution of approximately 1–3 million years (m.yr.) in the Phanerozoic and contain 4173 gap‐bound rock packages. Many important aspects of the North American geologic record are revealed by the temporal distribution of gap‐bound rock packages, including rock quantity, long‐term rates of sediment accumulation, and basin turnover. The durations of gap‐bound sedimentary successions are approximately exponentially distributed, with a mean duration of 25.2 m.yr. and a median duration of 16.9 m.yr. The probability of initiation and truncation among sedimentary packages does not increase or decrease substantially during the Phanerozoic, but these parameters do vary on shorter timescales in response to tectonically and glacioeustatically driven changes in sea level. The largest increase in the rate of sediment truncation occurs at the end‐Permian, which marks a clear and fundamental temporal discontinuity in the sedimentary record of North America. Smaller discontinuities occur at the end‐Ordovician, the end‐Triassic, and the end‐Cretaceous. Lithologically, Cambrian‐Mississippian sedimentary successions are dominated by carbonates, and post‐Paleozoic successions are dominated by terrigenous clastics. The quantity of preserved rock, the carbonate/siliciclastic ratio, and the dominant lithology comprising terrigenous clastics all vary substantially from interval to interval during the Phanerozoic, indicating that processes governing the formation and destruction of sedimentary rocks vary on timescales of <5 m.yr.

Climate change and the selective signature of the Late Ordovician mass extinction

Selectivity patterns provide insights into the causes of ancient extinction events. The Late Ordovician mass extinction was related to Gondwanan glaciation; however, it is still unclear whether elevated extinction rates were attributable to record failure, habitat loss, or climatic cooling. We examined Middle Ordovician-Early Silurian North American fossil occurrences within a spatiotemporally explicit stratigraphic framework that allowed us to quantify rock record effects on a per-taxon basis and assay the interplay of macrostratigraphic and macroecological variables in determining extinction risk. Genera that had large proportions of their observed geographic ranges affected by stratigraphic truncation or environmental shifts at the end of the Katian stage were particularly hard hit. The duration of the subsequent sampling gaps had little effect on extinction risk, suggesting that this extinction pulse cannot be entirely attributed to rock record failure; rather, it was caused, in part, by habitat loss. Extinction risk at this time was also strongly influenced by the maximum paleolatitude at which a genus had previously been sampled, a macroecological trait linked to thermal tolerance. A model trained on the relationship between 16 explanatory variables and extinction patterns during the early Katian interval substantially underestimates the extinction of exclusively tropical taxa during the late Katian interval. These results indicate that glacioeustatic sea-level fall and tropical ocean cooling played important roles in the first pulse of the Late Ordovician mass extinction in Laurentia.